1. Field of the Invention
This invention relates to a radiation image read-out apparatus used in a radiation image recording and reproducing system.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object to have a radiation image stored therein and is then scanned with stimulating rays such as a laser beam which cause it to emit light in the pattern of the stored image. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as a photographic film or on a display device such as a cathode ray tube (CRT).
One embodiment of the aforesaid radiation image recording and reproducing system is disclosed, for example, in Japanese Unexamined Patent Publication No. 58(1983)-67243. In the embodiment, before final read-out for scanning the stimulable phosphor sheet carrying a radiation image of an object stored therein by stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the radiation energy stored, detecting the emitted light by a photoelectric read-out means and converting it into an electric image signal is conducted, preliminary read-out for approximately detecting the image information stored in the stimulable phosphor sheet is conducted by use of stimulating rays of a level lower than the level of the stimulating rays used in the final read-out. Read-out conditions for the final read-out are adjusted on the basis of the information obtained by the preliminary read-out, and the final read-out is conducted by use of the read-out conditions.
FIG. 4 is a perspective view showing an example of the conventional read-out apparatus used in the aforesaid radiation image recording and reproducing system, wherein the read-out apparatus for the final read-out may be used also for the preliminary read-out.
In FIG. 4, preliminary read-out is first conducted on a stimulable phosphor sheet carrying a radiation image stored therein. A laser beam 3 emitted by a laser beam source 2 is passed through a beam expander 4, and the beam diameter of the laser beam 3 is strictly adjusted. Then, the laser beam 3 is passed through an ND filter 5, a prism 6 and a concave lens 7 to reduce the strength of the laser beam 3 and increase the beam diameter thereof. Also, the optical path is shifted away from a light input face of a light guide member 12 so that a part of the expanded laser beam is not intercepted by the light guide member 12. Then, the laser beam 3 is one-dimensionally deflected by a light deflector 8 constituted by a galvanometer mirror or the like and is made to impinge upon a stimulable phosphor sheet 1 by a plane reflection mirror 9. Between the light deflector 8 and the plane reflection mirror 9 is positioned an f.theta. lens 10 for maintaining the beam diameter of the laser beam 3 uniform during the scanning of the laser beam 3 on the sheet 1. While the laser beam 3 impinges upon the stimulable phosphor sheet 1, the sheet 1 is moved in the direction as indicated by the arrow 11 (sub-scanning direction) and, consequently, the whole area of the sheet 1 is exposed to and scanned by the laser beam 3. The power of the laser beam source 2, the beam diameter of the laser beam 3, the scanning speed of the laser beam 3, and the moving speed of the stimulable phosphor sheet 1 are selected so that the stimulation energy of the laser beam 3 for the preliminary read-out is smaller than the stimulation energy of the laser beam for the final read-out. When exposed to the laser beam 3, the stimulable phosphor sheet 1 emits light in proportion to the stored radiation energy, and the emitted light enters the light guide member 12. The light guide member 12 has a linear light input face positioned close to the scanning line on the stimulable phosphor sheet 1, and a ring-shaped light output face in close contact with the light receiving face of a photodetector 13, which may be a photomultiplier. The light guide member 12 is fabricated of a transparent thermoplastic resin sheet such as an acrylic resin sheet so that the light entering from the light input face can be transmitted to the light output face by total reflection inside of the light guide member 12. The light emitted by the stimulable phosphor sheet 1 upon stimulation thereof is guided inside of the light guide member 12, emitted from the light output face of the light guide member 12 and received by the photodetector 13. The light guide member 12 may be of a shape and a material as disclosed in U.S. Pat. No. 4,346,295.
The light receiving face of the photodetector 13 is provided with a filter (not shown) for transmitting only the light having the wavelength distribution of the light emitted by the stimulable phosphor sheet 1 and cutting off the light having the wavelength distribution of the stimulating rays, so that the photodetector 13 can detect only the light emitted by the stimulable phosphor sheet 1 upon stimulation thereof. The output of the photodetector 13 is sent to an amplifier 15 for the preliminary read-out via a switch 14 and is amplified thereby. The image information generated by the amplifier 15 is sent to a control circuit 16.
On the basis of the image information thus obtained, the control circuit 16 adjusts an amplification degree setting value (a) and a scale factor setting value (b) used as final read-out conditions, and an image processing condition setting value (c).
When the preliminary read-out is finished, the stimulable phosphor sheet 1 is returned to the read-out starting position 17 by a reverse rotation of a sub-scanning motor (not shown), and then the final read-out is started. In the final read-out, the laser beam 3 is emitted by the laser beam source 2 in the same manner as in the preliminary read-out. However, in the final read-out, the ND filter 5, the prism 6 and the concave lens 7 are moved away from the optical path in the direction as indicated by the arrow 18. Therefore, after the beam diameter of the laser beam 3 is strictly adjusted by the beam expander 4, the laser beam 3 directly reaches the light deflector 8 constituted by a galvanometer mirror or the like, is one-dimensionally deflected thereby, and is made to impinge upon the sheet 1 via the plane reflection mirror 9. The f.theta. lens 10 again maintains the scanning speed of the laser beam 3 constant during the scanning of the laser beam 3 on the sheet 1. While the laser beam 3 impinges upon the stimulable phosphor sheet 1, the sheet 1 is moved at a predetermined speed in the direction as indicated by the arrow 11 (sub-scanning direction) and consequently the whole area of the sheet 1 is exposed to the laser beam 3. Upon exposure to the laser beam 3, the stimulable phosphor sheet 1 emits light in proportion to the stored radiation energy. The emitted light enters the light guide member 12 and is detected by the photodetector 13 in the same manner as in the preliminary read-out. The output of the photodetector 13 is sent through the switch 14 to a final read-out amplifier 19 the sensitivity of which has been adjusted by the amplification degree setting value (a), and amplified to an appropriate level. The amplified electric signal is sent to an A/D converter 20, in which it is converted to a digital signal by use of a scale factor which has been adjusted by the scale factor setting value (b) to suit the width of signal fluctuation. The digital signal thus obtained is sent to the signal processing circuit 21, in which it is processed based on the image processing condition setting value (c) so as to obtain a radiation image suitable for viewing, particularly for diagnostic purposes. The signal is then sent to an image reproducing apparatus for reproducing a visible radiation image.
FIG. 5 is a perspective view showing a reflection mirror section at which a reflection mirror is positioned facing the light guide member in the read-out apparatus of FIG. 4. The light emitted by the stimulable phosphor sheet when it is exposed to stimulating rays is non-directional and weak. Therefore, as shown in FIG. 5, in order to improve the light guiding efficiency, it has been proposed in U.S. Pat. No. 4,346,295 to position a reflection mirror 38 on the side opposite to the light input face of the light guide member 12 with respect to the scanning line on the stimulable phosphor sheet 1. The reflection surface of the reflection mirror 38 stands face to face with the light input face of the light guide member 12, and has a length approximately equal to the length of the light input face.
However, in order to improve the light guiding efficiency as much as possible, the reflection mirror 38 is positioned as close to the light input face of the light guide membet 12 as possible, for example, at a spacing of t=1 mm from the light input face. Therefore, when the beam diameter of the stimulating rays 3 is increased for the preliminary read-out and the optical path is shifted as described above, there arises the problem that the stimulating rays 3 are intercepted by the reflection mirror.